McDonnell & Miller 42S Controller

The McDonnell & Miller 42S Controller (Part Number 129302) is a critical float-operated combination low water cut-off (LWCO) and pump controller designed for residential, commercial, and industrial steam boilers. Engineered for systems with a separate water column, this steam boiler water level control features dual SPST snap-action switches that provide reliable automatic reset functionality. The 129302 pump controller handles a maximum operating pressure of 50 psi (3.5 kg/cm²), making it a standard for medium-pressure steam applications. For durability in harsh environments, it utilizes a corrosion-resistant Monel bellows and includes an enclosed junction box to protect internal electrical components. This mechanical water level control is a direct replacement for discontinued mercury switch models, offering a safer, non-hazardous switching solution without sacrificing performance. Proper installation requires 1" FIP connections and, for higher amperage loads, should be used with a magnetic starter or relay to ensure longevity of the snap-action mechanisms.

• Dual-Function Reliability: Integrated LWCO and pump control ensures consistent water levels while protecting the burner during low-water conditions.
• Rugged Construction: Monel bellows assembly resists corrosion and scaling, extending service life in demanding steam cycles.
• Seamless Integration: Designed for boilers of any steaming capacity as long as a separate water column is present.
• Environmental Compliance: Features modern snap-switch technology to replace legacy mercury-filled controllers.
• Ease of Maintenance: Enclosed junction box allows for straightforward wiring and protection against dust and moisture.

Can the 42S replace an older Model 42 mercury switch controller?
Yes, the 42S is the direct non-mercury replacement for the legacy Model 42. It fits the same footprint and provides the same low water and pump control sequencing.

What is the maximum pressure rating for the 129302?
This unit is rated for a maximum steam pressure of 50 psi. For higher pressure requirements, you should specify a Series 150 or 157 controller.

Does this unit require a manual reset after a low-water event?
No, the 42S (129302) is equipped with an automatic reset. Once water returns to a safe operating level, the burner circuit will re-energize automatically.

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Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.